Hello everyone,
When setting up a VOI for small volume correction the VOI seems not be masked with the mask image from the whole-brain GLM for some of the calculations. For illustrative purpose:
Whole-brain analysis: 161,479 voxels = 955.7 resels
SVC based on the mask image: 161,479 voxels = 955.7 resels
SVC based on a spherical VOI with radius = 100 mm: 161,173 voxels = 3,334.2 resels
SVC based on a spherical VOI with radius = 200 mm: 161,479 voxels = 26,673.6 resels
SVC based on a spherical VOI with radius = 1,000 mm: 161,479 voxels = 3,334,200 resels
SVC based on a spherical VOI with radius = 2,000 mm: 161,479 voxels = 26,673,631.6 resels
Thus, with increasing size of the VOI the number of voxels does not exceed that of the whole-brain analysis, but the number of resels increases, and "Expected number of clusters", FWEc, and FWE-corrected cluster p values are affected. The current radii are meaningless, but
1) one might indeed go with some large sphere to e.g. cover the occipital cortex
2) sometimes spheres are placed near the border of the mask image (frontal eye fields)
with some parts of or even most of the sphere containing no data. This is somewhat problematic, it would be better to estimate the resel count based on the intersection of the VOI and the mask image (which seems to be done for some of the statistics anyway, e.g. the no. of voxels does not increase, nor does the FWEp).
Best
Helmut
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